Electrochemical oxidation of solutes in water can occur through two different oxidation mechanisms—direct oxidation and indirect oxidation. Direct oxidation involves two steps: (1) diffusion of solutes from the bulk solution to the anode surface and (2) oxidation of solutes at the anode surface. During indirect electrochemical oxidation, a strong oxidizing agent is electrochemically generated at the anode surface and oxidizes the target solutes. The indirect oxidation mechanism is more favourable in processes involving dilute solutions and is used frequently in the water treatment applications.
The electrolytic generation of chlorine species has particular advantages in the disinfection of drinking water, the principal one being that onsite generation of the chlorine eliminates the transport, handling and the storage of dangerous chlorine gas or the hazardous concentrated hypochlorite. The electro-generation of chlorine process is safe, environmentally friendly, easily operated and known to inactivate a wide range of micro-organisms ranging from bacteria to viruses and algae, the primary function of a disinfectant. Chlorine remains a predominant method for disinfecting drinking water as it provides both the primary and secondary functions despite the disadvantages of unfavourable taste and odour and the generation of potentially toxic chlorinated organic chlorination by-products. Alternative processes developed to overcome these disadvantages generally do not meet the secondary function of providing a residual protection in the distribution system. Active chlorine is traditionally introduced to water in the form of hypochlorite or chlorine gas. More recently electrochemical generation of hypochlorite has been advocated.
While electrochemical techniques offer attractive possibilities for a variety of treatments of water and wastewater, including electro-chlorination and electro-oxidation as well as electrolytic stripping and recovery of metals, they are generally too inefficient to be economically viable—particularly because of the high cost of electrodes, high voltages and high power consumption.
At low concentrations the rate of desired electrode reaction is limited by the rate at which the electro active species can diffuse to the electrode. In aqueous solutions, the predominant competing reaction at the electrodes will be electrolysis involving either H+ or OH− ions which will be maintained at constant concentration by the dissociation of water.
It is an object of the present invention to address the foregoing problems or at least to provide the public with a useful choice.
All references, including any patents or patent applications cited in this specification are hereby incorporated by reference. No admission is made that any reference constitutes prior art. The discussion of the references states what their authors assert, and the applicants reserve the right to challenge the accuracy and pertinency of the cited documents. It will be clearly understood that, although a number of prior art publications are referred to herein, this reference does not constitute an admission that any of these documents form part of the common general knowledge in the art, in New Zealand or in any other country.
Throughout this specification, the word “comprise”, or variations thereof such as “comprises” or “comprising”, will be understood to imply the inclusion of a stated element, integer or step, or group of elements integers or steps, but not the exclusion of any other element, integer or step, or group of elements, integers or steps.
Further aspects and advantages of the present invention will become apparent from the ensuing description which is given by way of example only.